The synthesis of developmental and evolutionary biology has become a major focus of research over the pas several years. However, the role of allele frequency changes at regulatory loci during short-term evolution is poorly understood. In this project I will exploit a special opportunity to investigate these issues afforded by the rapid and extensive morphological evolution among populations of threespine stickleback. When this boney fish has invaded freshwater habitats it has often evolved divergent body, head and jaw shapes in order to exploit different resources, and concurrently lost boney plates and spines. I will construct a genetic map or stickleback and identify quantitative trait loci (QTLs) by using a cross between ancestral marine and derived freshwater forms. Comparative genomics with the well studied zebrafish will allow me to define the molecular basis of the development-pathways leading to divergent morphologies. In situ expression analyses will then be performed to link changes in adult phenotype to changes in expression patterns during embryogenesis. The synthesis of zebrafish and stickleback research will greatly increase our understanding of the microevolution of development. A fuller understanding of the microevolution of development promises significant gains for human health. Higher rates of genetic disorders in some human populations can be better understood using models of the evolution of development, and population specific treatments can be devised using insight gained from these models. Additionally, more predictive models of the developmental evolution of infectious diseases will allow targeted treatments that will decrease the probability of epidemics of drug resistant pathogens.